U.S. patent application number 10/239954 was filed with the patent office on 2004-07-08 for device for producing hydrogen and method of operating the same.
Invention is credited to Fujii, Yasuhiro, Hosaka, Masato, Kitagawa, Koichiro, Shono, Toshiyuki, Taguchi, Kiyoshi, Tomizawa, Takeshi, Ukai, Kunihiro, Yoshida, Yutaka.
Application Number | 20040131540 10/239954 |
Document ID | / |
Family ID | 18603928 |
Filed Date | 2004-07-08 |
United States Patent
Application |
20040131540 |
Kind Code |
A1 |
Fujii, Yasuhiro ; et
al. |
July 8, 2004 |
Device for producing hydrogen and method of operating the same
Abstract
When a hydrogen producing apparatus is stopped, the flow rates
of a hydrocarbon-type fuel feedstock, water and an
oxygen-containing oxidant gas are decreased respectively. A random
decrease of the flow rates, however, invites a rapid increase in
the temperature of a catalyst beyond the limit of thermal
resistance, resulting in deactivation of the catalyst. Further,
this poses a danger, for example, that the residual
hydrocarbon-type fuel in the apparatus may be mixed with the
oxidant gas after the stopping of the apparatus. Thus, in stopping
the operation, the present invention exerts such control as to
decrease the flow rate of the hydrocarbon-type fuel and
simultaneously increase the flow rate of water while maintaining
the flow rate of the oxidant gas at a constant level, stop the
hydrocarbon-type fuel, and thereafter stop the water and the
oxidant gas.
Inventors: |
Fujii, Yasuhiro;
(Tokuyama-shi, JP) ; Hosaka, Masato; (Osaka-shi,
JP) ; Tomizawa, Takeshi; (Ikoma-shi, JP) ;
Ukai, Kunihiro; (Ikoma-shi, JP) ; Taguchi,
Kiyoshi; (Osaka-shi, JP) ; Shono, Toshiyuki;
(Soraku-gun, JP) ; Yoshida, Yutaka; (Nabari-shi,
JP) ; Kitagawa, Koichiro; (Fujisawa-shi, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Family ID: |
18603928 |
Appl. No.: |
10/239954 |
Filed: |
January 8, 2003 |
PCT Filed: |
March 23, 2001 |
PCT NO: |
PCT/JP01/02375 |
Current U.S.
Class: |
423/650 ;
422/105; 422/211 |
Current CPC
Class: |
B01J 21/04 20130101;
C01B 2203/1082 20130101; C01B 2203/1241 20130101; C01B 2203/107
20130101; B01J 2208/00707 20130101; C01B 2203/0844 20130101; C01B
2203/1052 20130101; C01B 2203/1614 20130101; C01B 3/38 20130101;
C01B 2203/0485 20130101; H01M 8/0612 20130101; C01B 2203/1064
20130101; B01J 8/0278 20130101; C01B 3/40 20130101; C01B 2203/169
20130101; C01B 2203/82 20130101; C01B 3/386 20130101; C01B 2203/045
20130101; C01B 2203/0811 20130101; C01B 2203/1276 20130101; B01J
23/40 20130101; B01J 8/0242 20130101; Y02E 60/50 20130101; C01B
2203/0244 20130101; C01B 2203/1609 20130101; B01J 8/001 20130101;
C01B 2203/1288 20130101; B01J 23/70 20130101; C01B 2203/1058
20130101; C01B 3/382 20130101; B01J 2208/00061 20130101; B01J
2208/00548 20130101; C01B 2203/1604 20130101; C01B 2203/142
20130101; C01B 2203/066 20130101; Y02P 20/52 20151101; C01B
2203/1076 20130101; B01J 37/0201 20130101 |
Class at
Publication: |
423/650 ;
422/211; 422/105 |
International
Class: |
C01B 003/24; B01J
008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2000 |
JP |
2000-87996 |
Claims
1. A method of operating a hydrogen producing apparatus for a fuel
cell power generation system, said apparatus producing a
hydrogen-containing gas by catalytic reaction among feedstocks
comprising at least a hydrocarbon-type fuel, water and an
oxygen-containing oxidant gas, said method comprising, in stopping
operation of the apparatus, the steps of: decreasing the flow rate
of the hydrocarbon-type fuel and simultaneously increasing the flow
rate of the water while maintaining the flow rate of the oxidant
gas at a constant level; stopping supply of the hydrocarbon-type
fuel; and thereafter stopping supply of the water and the oxidant
gas.
2. A hydrogen producing apparatus comprising: a reformer comprising
a reforming catalyst layer, a pre-mixing chamber and a vaporizing
chamber, each chamber being provided upstream of the reforming
catalyst layer; a supply unit of a hydrocarbon-type fuel and a
supply unit of an oxygen-containing oxidant gas, each unit having a
flow rate adjusting device and being connected to said pre-mixing
chamber; a supply unit of water having a flow rate adjusting device
and being connected to said vaporizing chamber; and a control unit
for controlling the respective flow rate adjusting devices, wherein
in stopping operation of the apparatus, said control unit controls
said respective flow rate adjusting devices in a procedure
comprising the steps of: decreasing the flow rate of the
hydrocarbon-type fuel and simultaneously increasing the flow rate
of the water while maintaining the flow rate of the oxidant gas at
a constant level; stopping supply of the hydrocarbon-type fuel; and
thereafter stopping supply of the water and the oxidant gas.
3. The hydrogen producing apparatus in accordance with claim 2,
further comprising a temperature detector for detecting upstream
temperature of said catalyst layer, wherein said control unit
exerts such control, in stopping the operation of the apparatus, as
to increase the rate of increase in the flow rate of the water when
the temperature detected by said temperature detector does not
decrease.
4. The hydrogen producing apparatus in accordance with claim 2,
further comprising a temperature detector for detecting upstream
temperature of said catalyst layer, wherein said control unit
exerts such control, in stopping the operation of the apparatus, as
to increase the rate of increase in the flow rate of the water when
the temperature detected by said temperature detector has reached a
predetermined upper limit.
5. The hydrogen producing apparatus in accordance with claim 2,
wherein said catalyst layer comprises at least platinum, and a
desulfurization unit comprising an oxide of at least one metal
selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu and
Zn as a desulfurizing agent is provided downstream of the
reformer.
6. The hydrogen producing apparatus in accordance with claim 2,
wherein the catalyst is supported on a carrier which comprises at
least one of zirconium oxide and aluminum oxide, and reforming
reaction is operated while the temperature of said catalyst layer
is held at 600 to 800.degree.C.
7. The hydrogen producing apparatus in accordance with claim 5,
wherein said desulfurizing agent is at least one selected from the
group consisting of V.sub.2O.sub.5, Cr.sub.2O.sub.3, MnO.sub.2,
Fe.sub.2O.sub.3, CO.sub.2O.sub.3, NiO, CuO and ZnO.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hydrogen producing
apparatus for use in a fuel cell power generation system that
generates electric power by reacting oxygen with hydrogen obtained
by reforming a hydrocarbon-type fuel and to an operating method
thereof.
BACKGROUND ART
[0002] Fuel cell power generation systems are expected to be highly
efficient energy conversion systems for consumer use. A
representative fuel cell power generation system has a hydrogen
producing apparatus incorporated therein for producing hydrogen
from an easily available hydrocarbon-type fuel such as natural gas,
LPG, alcohol or kerosene by catalytic reaction. One of hydrogen
production methods is an auto-thermal method in which a reformer
having a catalyst charged therein is supplied with a
hydrocarbon-type fuel, water and an oxygen-containing oxidant gas
such as air to cause a steam reforming reaction of the
hydrocarbon-type fuel and an oxidation reaction of the hydrocarbon
to proceed simultaneously. As a result, hydrogen is produced mainly
by the steam reforming reaction. Since the steam reforming reaction
is an endothermic reaction which proceeds at high temperatures of
about 700.degree. C. or higher, the oxidation reaction, which is an
exothermic reaction, is utilized to obtain necessary heat. The
auto-thermal method is characterized in that it requires a
relatively short time for starting up and stopping the apparatus.
However, ensuring safety in frequent start-up and stopping is
required in consumer products such as domestic cogeneration systems
unlike power plants and other facilities to be operated
steadily.
[0003] When the hydrogen producing apparatus is stopped, the flow
rates of feedstock hydrocarbon-type fuel, water and oxidant gas are
respectively decreased. A random decrease of these flow rates,
however, will cause a problem that the temperature of a catalyst
rises quickly beyond the limit of thermal resistance, resulting in
deactivation of the catalyst. Another problem is that there is a
danger, for example, that the residual hydrocarbon in the apparatus
may be mixed with the oxidant gas after the stopping of the
apparatus.
[0004] Also, the hydrocarbon-type fuel to be used as the feedstock
for such a hydrogen producing apparatus contains small amounts of
sulfur compounds. Thus, if such a feedstock is directly introduced
to the reformer, it poisons the reforming catalyst, CO shift
catalyst or the like, leading to performance deterioration.
Therefore, as a method for removing the above-described sulfur
compounds, there is proposed a method in which an oxide of
transition metals such as zinc oxide, zeolite or the like is
arranged upstream of the reforming catalyst for desulfurizing the
sulfur compounds. There are a chemical desulfurization method and a
physical adsorption method with regard to the desulfurization
method. In a chemical desulfurization reaction using a metal oxide,
the metal oxide needs to be retained at a high temperature of about
400.degree. C. Also, in desulfurization using a physical adsorbent
such as zeolite, adsorption/replacement takes place due to steam
contained in the feedstock gas or the like to cause elimination of
sulfur compounds from the feedstock gas, possibly poisoning the
catalyst positioned downstream. Further, in the case of using a
zeolite-type adsorbent, the volume of the adsorbent relative to the
desulfurizing capability is large in comparison with the chemical
reaction desulfurization using zinc oxide or the like, and this
becomes a problem in making the apparatus more compact. Therefore,
it is desired to develop a method in which chemical desulfurization
using a metal oxide is performed at a rear part of the reforming
section or downstream of the reforming section using an existing
heat source of the CO shifting section. Therefore, there is an
urgent need to establish a reforming catalyst having durability
with respect to sulfur compounds and catalyst operating
conditions.
[0005] In view of the above points, an object of the present
invention is to provide a hydrogen producing apparatus that is
capable of decreasing the flow rates of feedstocks without inviting
a rapid increase in catalyst temperature and is capable of stopping
the apparatus without leaving a residual hydrocarbon-type fuel
after the stopping.
[0006] Another object of the present invention is to provide a
hydrogen producing apparatus that is capable of effective
desulfurization downstream of a reformer.
DISCLOSURE OF INVENTION
[0007] The present invention is directed to a method of operating a
hydrogen producing apparatus for a fuel cell power generation
system, the apparatus producing a hydrogen-containing gas by
catalytic reaction among feedstocks comprising at least a
hydrocarbon-type fuel, water and an oxygen-containing oxidant gas,
the method comprising, in stopping operation of the apparatus, the
steps of: decreasing the flow rate of the hydrocarbon-type fuel and
simultaneously increasing the flow rate of the water while
maintaining the flow rate of the oxidant gas at a constant level;
stopping the hydrocarbon-type fuel; and thereafter stopping the
water and the oxidant gas.
[0008] The present invention relates to a hydrogen producing
apparatus comprising: a reformer comprising a reforming catalyst
layer, a pre-mixing chamber and a vaporizing chamber, each chamber
being provided upstream of the reforming catalyst layer; a supply
unit of a hydrocarbon-type fuel and a supply unit of an
oxygen-containing oxidant gas, each unit having a flow rate
adjusting device and being connected to the pre-mixing chamber; a
supply unit of water having a flow rate adjusting device and being
connected to the vaporizing chamber; and a control unit for
controlling the respective flow rate adjusting devices, wherein in
stopping operation of the apparatus, the control unit controls the
respective flow rate adjusting devices in a procedure comprising
the steps of: decreasing the flow rate of the hydrocarbon-type fuel
and simultaneously increasing the flow rate of the water while
maintaining the flow rate of the oxidant gas at a constant level;
stopping supply of the hydrocarbon-type fuel; and thereafter
stopping supply of the water and the oxidant gas.
[0009] In a preferable mode, the apparatus further comprises a
temperature detector for detecting upstream temperature of the
catalyst layer, wherein the control unit exerts such control, in
stopping the operation of the apparatus, as to increase the rate of
increase in the flow rate of the water when the temperature
detected by the temperature detector does not decrease.
[0010] In another preferable mode of the present invention, the
hydrogen producing apparatus further comprises a temperature
detector for detecting upstream temperature of the catalyst layer,
wherein the control unit exerts such control, in stopping the
operation of the apparatus, as to increase the rate of increase in
the flow rate of the water when the temperature detected by the
temperature detector has reached a predetermined upper limit.
[0011] It is preferred that the upper limit be not less than
900.degree. C. and not more than 1200.degree.C.
[0012] It is preferred that the reformer comprise a reforming
catalyst layer comprising at least platinum and that a
desulfurization unit comprising an oxide of at least one metal
selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu and
Zn be provided downstream of the reformer.
[0013] It is desirable that the reforming catalyst be supported on
a carrier comprising at least one of zirconium oxide and aluminum
oxide and that reforming reaction be operated while the temperature
of the catalyst layer is held at 600 to 800.degree.C.
[0014] It is preferred that the desulfurization unit comprise a
desulfurizing agent which is at least one selected from the group
consisting of V.sub.2O.sub.5, Cr.sub.2O.sub.3, MnO.sub.2,
Fe.sub.2O.sub.3, CO.sub.2O.sub.3, NiO, CuO and ZnO.
BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1 is a structural diagram of a hydrogen producing
apparatus in Embodiment 1 of the present invention.
[0016] FIG. 2 is a structural diagram of a hydrogen producing
apparatus in Embodiment 2 of the present invention.
[0017] FIG. 3 is a chart showing changes with time in upstream
temperature of catalyst layers of hydrogen producing apparatuses in
an example of the present invention and comparative examples.
BEST MODE FOR CARRYING OUT THE INVENTION
[0018] In the following, embodiments of the present invention will
be described with reference to drawings.
[0019] Embodiment 1
[0020] FIG. 1 shows the constitution of a hydrogen producing
apparatus in one embodiment of the present invention. In FIG. 1,
numeral 11 denotes a reformer which has a reforming catalyst
charged therein and produces a hydrogen-containing gas by the
reaction of the catalyst at high temperatures. The reformer 11 has,
in the order of top to bottom, an outlet 22 of hydrogen-containing
gas from which a hydrogen-containing gas produced is collected, a
reforming catalyst layer 20 and a vaporizing chamber 13, and it
also has a pre-mixing chamber 12 at its side face between the
reforming catalyst layer 20 and the vaporizing chamber 13. The
pre-mixing chamber 12 is connected to a supply conduit 14 of
hydrocarbon and a supply conduit 15 of air serving as an oxidant
gas and is a place where hydrocarbon and air supplied are mixed
with each other. An igniter 18 composed of electric heater wires
and a catalyst-preheating combustor 19 are used to heat the
catalyst layer 20 and the vaporizing chamber 13 in starting up the
apparatus. The hydrocarbon supply conduit 14 and the air supply
conduit 15 are provided with a proportional valve 14a capable of
adjusting the flow rate of hydrocarbon and a blower 15a capable of
adjusting the flow rate of air, respectively. A water supply
conduit 16 connected to the vaporizing chamber is provided with a
pump 16a capable of adjusting the flow rate of water (the flow rate
of steam). A control unit 17 is a controller comprising electric
circuit which controls auxiliaries including the proportional valve
14a, blower 15a, pump 16a and igniter 18.
[0021] The main structure of the hydrogen producing apparatus is
composed of stainless steel having thermal resistance, and the
reformer 11 is composed of stainless steel of which outer walls are
coated with a heat insulating material 21. The catalyst layer 20 is
composed of activated particles of platinum group metal such as
platinum, palladium, rhodium or ruthenium which are supported on a
ceramic carrier. When the hydrogen producing apparatus of this
embodiment is operated steadily, a mixed gas of hydrocarbon, air
and steam is supplied to the catalyst layer 20 to cause a steam
reforming reaction and an oxidation reaction to proceed, and a
hydrogen-containing gas is collected from the outlet 22. The
temperature of the catalyst layer is usually kept at 700.degree. C.
or higher.
[0022] The operating conditions of this apparatus will be described
in the following. This embodiment uses methane which is a main
component of natural gas used for city gas as a hydrocarbon
feedstock. In starting up the apparatus, the proportional valve 14a
and the blower 15a are controlled by the control unit such that the
flow rate ratio of methane/air becomes approximately 1:10. For
example, methane and air are supplied at 1 L/min and 10 L/min,
respectively, and by actuating the igniter 18 comprising electric
heater wires, the catalyst-preheating combustor 19 is operated to
heat the catalyst layer 20 and the vaporizing chamber 13. After the
catalyst layer has reached a predetermined temperature at which the
reaction is to be commenced, a transition is made into a steady
operation. In one example of the operating conditions using
methane, the proportional valve 14a, the blower 15a and the pump
16a are controlled by the control unit 17 during the steady
operation such that the flow rate ratio of methane/air/steam
becomes 1:3:1. For example, methane, air and steam are supplied at
10 L/min, 30 L/min and 10 L/min, respectively. In this case, the
upstream temperature of the catalyst layer becomes about
740.degree. C. The gas produced during the steady operation
contains hydrogen, nitrogen, carbon monoxide and carbon dioxide in
a ratio of 8:8:1:2 on a dry basis, and it further contains small
amounts of residual methane and steam. The steady operation is a
continuous operation of approximately several hours under constant
conditions. In stopping the operation of the apparatus, the
proportional valve 14a, the blower 15a and the pump 16a are
controlled by the control unit 17 in such a manner as to gradually
decrease the flow rate of methane and simultaneously gradually
increase the flow rate of steam while maintaining the flow rate of
air at a constant level, stop supply of hydrocarbon-type fuel, and
thereafter stop supply of steam and air. Examples of the stopping
method of the operation will be described with reference to
comparative examples.
[0023] Embodiment 2
[0024] FIG. 2 shows the constitution of a hydrogen producing
apparatus in another embodiment of the present invention. Since the
constitution of the apparatus as shown in FIG. 2 is almost similar
to that of FIG. 1, only the differences will be described. This
apparatus is provided with a thermocouple 31 which detects upstream
temperature of the catalyst layer and send signals of voltage value
to the control unit 17. The operating conditions of this apparatus
are the same as those of Embodiment 1. The flow rates of
hydrocarbon, air and steam are controlled by the control unit 17.
The control unit 17 stores an algebraic expression for calculating
the rate of increase in the flow rate of steam from the rate of
decrease in the flow rate of hydrocarbon-type fuel on the basis of
experiment results. In stopping the operation, the control unit
gradually decreases the flow rate of hydrocarbon-type fuel while
maintaining the flow rate of air at a constant level; and
simultaneously gradually increases the flow rate of steam by
successively substituting the rate of decrease in the flow rate of
hydrocarbon-type fuel in the expression and using the calculated
results as the rate of increase in the flow rate of steam. Further,
the control unit 17 successively changes coefficients of the
expression depending on the upstream temperature of the catalyst
layer. When the upstream temperature of the catalyst layer does not
drop, i.e., when it rises or it does not change, the control unit
changes coefficients of the expression and thereby exerts control
to increase the rate of increase in the flow rate of steam.
[0025] As described above, since the changes of the flow rates of
hydrocarbon-type fuel and steam are allowed to reflect the upstream
temperature of the catalyst layer, this embodiment is effective
when fixing the expression would cause inconveniences. For example,
in the case of the catalytic activity deteriorating with time after
repetition of starting-up the apparatus and stopping the operation
over an extended period or in the case of variations in the
composition of the hydrocarbon feedstock, if the flow rates are
controlled with the expression fixed to the one obtained in an
early stage of the operation, the upstream temperature of the
catalyst layer may become higher than the temperature in the early
stage. The constitution of this embodiment makes it possible to
control the flow rate of steam depending on the catalytic activity
and reaction state and therefore prevent the temperature of the
catalyst layer from rising. The above-mentioned algebraic
expression may be, for example, as follows:
.DELTA.(HC).times.(aT+bT.sup.2+cT.sup.3)=.DELTA.(H.sub.2O)
[0026] wherein .DELTA. (HC) represents the amount of change in
hydrocarbon-type fuel, .DELTA. (H.sub.2O) represents the amount of
change in steam, and a, b and c are constants.
[0027] Embodiment 3
[0028] A hydrogen producing apparatus in still another embodiment
of the present invention will be explained. Since this embodiment
is almost similar to Embodiment 2, only the differences will be
described. The control unit 17 is made to store the upper limit of
catalyst operating temperature, for example, 900.degree.C. In
stopping the operation, the control unit gradually decreases the
flow rate of hydrocarbon-type fuel while maintaining the flow rate
of air at a constant level; and simultaneously gradually increases
the flow rate of steam by successively substituting the rate of
decrease in the flow rate of hydrocarbon-type fuel in the
expression and using the calculated results as the rate of increase
in the flow rate of steam. Further, the control unit 17
successively changes coefficients of the expression depending on
the upstream temperature of the catalyst layer. When the upstream
temperature of the catalyst layer has reached the above-mentioned
900.degree. C., the control unit changes coefficients of the
expression and thereby increases the rate of increase in the flow
rate of steam. Furthermore, it is also possible to exert loop
control of the flow rate of steam for adjustment based on the
upstream temperature of the catalyst layer; as a result, it is
possible, by increasing the rate of increase in the flow rate of
steam, to exert control that the temperature does not rise far
beyond the upper limit of the operating temperature. The
constitution of this embodiment makes it possible to ensure
prevention of catalyst deterioration caused by heat in stopping the
apparatus.
[0029] In the following, examples of stop operation of the
apparatus of the present invention will be described with reference
to comparative examples.
EXAMPLE 1
[0030] FIG. 3 is a chart showing changes in upstream temperature of
catalyst layers. In FIG. 3, the abscissa is the time from the
commencement of the stop operation and the ordinate is the upstream
temperature of catalyst layers.
[0031] From the steady operation as described in Embodiment 1,
first, the flow rate of methane was gradually decreased from the
steady-state value of 10 L/min in a ratio of 0.2 L/min per second
while the flow rate of air was maintained at the steady-state value
of 30 L/min, and simultaneously the flow rate of steam was
gradually increased from the steady-state value of 10 L/min in a
ratio of 0.2 L/min per second. Fifty seconds after the commencement
of the stop operation, methane was stopped. Ten seconds after the
stop of methane, steam was stopped, and five minutes later, air was
stopped. As shown by "a" of FIG. 3, this stop operation
successfully stopped the operation without causing an abnormal rise
in upstream temperature of the catalyst layer and without affecting
the catalyst adversely. Further, since air was supplied even after
the stop of methane, this stop operation prevented the flammable
gas from remaining inside the apparatus.
COMPARATIVE EXAMPLE 1
[0032] A stop operation different from that of Example 1 will be
described as a comparative example. From the steady operation as
described in Embodiment 1, while the flow rates of air and steam
were maintained at 30 L/min and 10 L/min, respectively, the flow
rate of methane was gradually decreased in the same ratio of 0.2
L/min per second as that of Example 1 and was stopped fifty seconds
later. Ten seconds after the stop of methane, steam was stopped,
and five minutes later, air was stopped. This stop operation caused
a rapid increase in the upstream temperature of the catalyst layer
as shown by "b" of FIG. 3. The reason may be as follows: since the
amount of oxidation reaction (the amount of combustion) defined by
the flow rate of air is maintained while the flow rate of methane
is reduced, this will cause an exothermic reaction equivalent to
that in the steady operation to proceed in a smaller area than in
the steady operation upstream of the catalyst layer over the
catalyst. Since the temperature of the catalyst exceeded
900.degree. C., the limit of the operating temperature, the
activity thereof was deteriorated due to sintering of catalyst
particles occurring mainly upstream of the catalyst layer.
Generally speaking, deterioration in catalytic activity due to such
cause is difficult to restore.
COMPARATIVE EXAMPLE 2
[0033] From the steady operation as described in Embodiment 1,
while the flow rate of steam was maintained at the steady-state
value, the flow rate of methane was gradually decreased from the
steady-state value of 10 L/min in a ratio of 0.2 L/min per second,
and simultaneously the flow rate of air was gradually increased
from the steady-state value of 30 L/min in a ratio of 0.2 L/min per
second. Fifty seconds after the commencement of the stop operation,
methane was stopped. Ten seconds after the stop of methane, steam
was stopped, and five minutes later, air was stopped. This stop
operation also caused a rapid increase in the upstream temperature
of the catalyst layer as shown by "c" of FIG. 3. The reason may be
as follows: That is, the flow rate of air during the steady
operation is low as compared to the rate designed for complete
combustion (oxidation reaction) of methane. Since the oxidation
reaction has a higher reaction rate than the steam reforming
reaction of methane, the amount of oxidation reaction was increased
immediately after the commencement of the stop operation by the
increase in the flow rate of air, and the exothermic reaction was
therefore increased as compared with the steady operation. Since
the temperature of the catalyst exceeded, in a larger area, the
temperature limit of 900.degree.C., the activity thereof was
deteriorated due to sintering of catalyst particles.
COMPARATIVE EXAMPLE 3
[0034] From the steady operation as described in Embodiment 1,
while the flow rate of methane was maintained at the steady-state
value, the flow rate of air was decreased in a ratio of 0.6 L/min
per second, and simultaneously the flow rate of steam was increased
in a ratio of 0.6 L/min per second, and fifty seconds later, air
was stopped. Five minutes after the stop of air, steam was stopped,
and ten minutes later, methane was gradually stopped. As shown by
"d" of FIG. 3, this stop operation successfully stopped the
operation without causing an abnormal temperature rise and without
affecting the catalyst adversely. However, methane remained in the
apparatus, and if air is introduced for removing the methane, there
is a danger that a flammable air-fuel mixture may be produced.
[0035] As indicated in the foregoing example and comparative
examples, there may be an adverse effect on the catalyst or a
danger unless the apparatus is stopped by gradually decreasing the
flow rate of methane and simultaneously gradually increasing the
flow rate of steam while maintaining the flow rate of air at a
constant level.
[0036] It is noted that the rate of increase in the flow rate of
steam needs to compensate for the rate of decrease in the flow rate
of methane. The difference between Example 1 and Comparative
example 1 is that the flow rate of steam is raised or maintained at
a constant level. In Comparative example 1, the catalyst
temperature rose rapidly, presumably because decreasing only the
flow rate of methane without increasing the flow rate of steam
caused a change in the total flow rate of the gases and the amount
of reaction, thereby changing heat balance over the catalyst. The
rate of increase in the flow rate of steam is determined by
substituting the rate of decrease in the flow rate of
hydrocarbon-type fuel in a predetermined expression, and the
predetermined expression is determined so as to compensate for such
changes. In Example 1, it was determined, as an example, to be
equal to the rate of decrease in the flow rate of methane. It is
noted, however, that the conditions of this example do not
necessarily apply to all cases and may be modified according to
conditions such as the amount, material and shape of a catalyst,
the thermal capacity, material and shape of an apparatus, the flow
rates of gases, etc. for carrying out the invention. For example,
the predetermined expression is determined based on experiments
with actual apparatuses and simulation, and the control of the flow
rates is performed based on the determined expression. It is noted
that the control of the flow rates may be performed manually or
with the use of the control unit 17.
EXAMPLE 2
[0037] This example examined the optimal reforming catalyst,
reforming reaction conditions and desulfurizing agent to be used in
a hydrogen producing apparatus having a constitution that sulfur
compounds contained in a hydrocarbon-type fuel are desulfurized by
a desulfurization unit placed downstream of a reforming
section.
[0038] First, the durability of the catalyst with respect to the
sulfur compounds was tested.
[0039] Each one of six (sic) kinds of elements, Pt, Pd, Rh, Ir, Ru,
Co, Ni and Cu, was carried on aluminum oxide at 3 wt % to prepare a
catalyst. A dinitrodiammine complex salt was used for Pt, and
nitrate was used for the other elements.
[0040] First, aluminum oxide was impregnated with a solution of a
salt of catalyst metal and was thermally decomposed at 500.degree.
C. for one hour so that the catalyst was carried. The resultant
alumina powder carrying catalyst metal was compression moulded and
crushed to form particles of 8 to 15 mesh.
[0041] The particles were charged into a quarts tube, and a mixed
gas of methane, steam and air was introduced into the quarts tube
at a space velocity of 10000 h.sup.-1. The molar ratio of
methane/steam/air was 1:3:2.5. Further, tertiary butyl mercaptan
(hereinafter referred to as TBM) and dimethyl sulfide (hereinafter
referred to as DMS), each being a component of an odorant of city
gas, were added thereto each at 2.5 ppm. The quarts tube was
inserted into a tubular furnace, and while the temperature of the
furnace was maintained at 800.degree. C., the change with time in
the conversion ratio of methane was observed. Before the
introduction of the mixed gas, the catalyst was supplied with He
containing 10% H.sub.2 at 400.degree. C. and at a space velocity of
10000 h.sup.-1 for one hour for reduction by hydrogen. This
evaluation is referred to as Experiment 1. The results are shown in
Table 1.
1 TABLE 1 Conversion ratio Conversion ratio 1 hour after 24 hours
after introduction of introduction of Catalyst feedstock gas (%)
feedstock gas (%) Pt/Al.sub.2O.sub.3 97.5 98.0 Pd/Al.sub.2O.sub.3
79.0 54.1 Rh/Al.sub.2O.sub.3 88.4 62.2 Ir/Al.sub.2O.sub.3 90.7 81.6
Ru/Al.sub.2O.sub.3 59.9 33.3 Co/Al.sub.2O.sub.3 55.2 45.5
Ni/Al.sub.2O.sub.3 68.5 46.0 Cu/Al.sub.2O.sub.3 30.9 32.6
[0042] Table 1 indicates that the catalyst comprising
Al.sub.2O.sub.3 and Pt carried thereon has higher durability with
respect to sulfur compounds than the other catalyst metals.
[0043] TBM and DMS in the mixed gas were converted to hydrogen
sulfide after they came in contact with the reforming catalyst.
Further, the desulfurization unit placed downstream of the
reforming section was filled with a desulfurizing agent
V.sub.2O.sub.5, Cr.sub.2O.sub.3, MnO.sub.2, Fe.sub.2O.sub.3,
Co.sub.2O.sub.3, NiO, CuO or ZnO having a volume five times that of
the reforming catalyst, and while the temperature of the
desulfurization unit was held at 400.degree. C., the desufurization
characteristics were examined. As a result, no sulfur component was
detected downstream of the desulfurization unit in the case of any
desulfurizing agents. Gas chromatography was used to detect the
sulfur component. The minimum sensitivity was 0.1 ppm.
[0044] With regard to the desulfurizing agent, it was found that
zinc oxide had the highest adsorption capacity and was therefore
appropriate as the desulfurizing agent. It was found that this
enabled desulfurization downstream of the reforming catalyst.
[0045] Next, the correlation between the composition of the
feedstock gas and the durability of the platinum catalyst with
respect to sulfur compounds was examined.
[0046] Experiments were conducted under the same conditions as
those of Experiment 1 except for changes in molar ratio of
feedstock gas methane/water/air. Table 2 shows changes with time in
methane conversion ratio when the composition of the mixed gas is
varied.
2 TABLE 2 Conversion ratio Conversion ratio 1 hour after 24 hours
after CH.sub.4:H.sub.2O:Air introduction of introduction of molar
ratio feedstock gas (%) feedstock gas (%) 1:3:1 60.0 49.3 1:3:2.5
97.5 98.0 1:3:3 97.7 97.5 1:1:2.5 67.0 66.4 1:2:2.5 76.9 74.2 1:1:1
62.8 49.7 1:3:0 55.5 40.0 1:0:3 63.5 59.8
[0047] As shown in Table 2, the higher the molar ratios of air and
water, the higher the conversion ratio, and the amount of air gave
a considerable effect on deterioration of the catalyst. Therefore,
in view of the activity, perf ormance deterioration, etc., it is
essential that the feedstock gas contain both water and air. Also,
in view of the methane conversion ratio and the operating
temperature of the catalyst, larger amounts of water and air are
more preferable.
[0048] Next, the correlation between the catalyst temperature and
the durability of the catalyst with respect to sulfur compounds was
examined. Using the catalyst comprising Al.sub.2O.sub.3 carrying 3
wt % Pt, this examination was conducted in the same manner as in
Experiment 1. At this time, the temperature of the electric furnace
was varied to six different temperatures of 500.degree. C.,
600.degree. C., 700.degree. C., 800.degree. C., 900.degree. C. and
1000.degree. C., and the upstream temperature of the catalyst
surface was measured. Table 3 shows changes with time in methane
conversion ratio when the catalyst temperature is varied.
3 TABLE 3 1 hour after introduction of 24 hours after introduction
feedstock gas of feedstock gas Temperature Conversion Catalyst
Conversion Catalyst of electric ratio temperature ratio temperature
furnace (.degree. C.) (%) (.degree. C.) (%) (.degree. C.) 500 48.4
576 10.4 581 600 80.0 599 58.8 624 700 91.7 617 91.7 620 800 97.5
677 98.0 678 900 98.5 750 97.9 775 1000 90.2 861 59.3 912
[0049] As shown in FIG. 3, when the catalyst temperature was in a
range of 600 to 800.degree. C., the conversion ratio remained high
and the catalyst deterioration was also suppressed.
[0050] Next, the effect of the catalyst carrier on the durability
of the platinum catalyst with respect to sulfur compounds was
examined. Five kinds of carriers, Al.sub.2O.sub.3, TiO.sub.2,
ZrO.sub.2, MgO and SiO.sub.2--Al.sub.2O.sub.3, were used and were
thermally treated in the air at 1000.degree. C. for one hour. Using
these carriers, experiments were conducted under the same
conditions as those of Experiment 1, and the changes with time in
methane conversion ratio are shown in Table 4.
4 TABLE 4 Conversion ratio Conversion ratio 1 hour after 24 hours
after introduction of introduction of Carrier feedstock gas (%)
feedstock gas (%) Al.sub.2O.sub.3 97.5 98.0 TiO.sub.2 72.2 68.1
ZrO.sub.2 99.0 98.9 MgO 83.9 80.5 SiO.sub.2-Al.sub.2O.sub- .3 74.6
51.4
[0051] As shown in Table 4, only Al.sub.2O.sub.3 and ZrO.sub.2
allow the catalyst to have high durability with respect to sulfur
compounds. A highly acidic carrier such as
SiO.sub.2--Al.sub.2O.sub.3 invited considerable deposition of coke,
and carriers having a low specific surface area and therefore poor
dispersion of the catalyst metal such as TiO.sub.2 and MgO also
exhibited poor durability.
[0052] In contrast, Al.sub.2O.sub.3 having a high specific surface
area and high dispersion exhibited high durability, and ZrO.sub.2
also exhibited high durability although it has a low specific
surface area and low dispersion. Stabilized zirconia ZrO.sub.2 to
which a rare-earth element was introduced also exhibited a similar
tendency.
INDUSTRIAL APPLICABILITY
[0053] As described above, the present invention is free from
deactivation of a catalyst caused by a rapid increase in the
temperature of the catalyst beyond the limit of thermal resistance
in stopping the operation. Therefore, it is possible to stop a
hydrogen producing apparatus without giving an adverse effect on
the catalyst and in a safe manner. Further, since there is no
residual hydrocarbon-type fuel inside the apparatus after stopping
the operation, it is possible to secure safety.
[0054] In the foregoing examples, methane was used as the
hydrocarbon-type fuel, but other hydrocarbon-type fuels may be used
or hydrocarbon component content may differ. Disclosed values and
ratios of the flow rates are just examples, and modifications
thereof may be possible. For example, city gas, LPG or the like may
have different components from region to region. In this case, the
flow rates of the above-described gases may be modified as
appropriate. Platinum group metals were used as the catalyst, but
there is no limitation thereto.
[0055] In the case of a problem of sulfur poisoning arising
depending on the kind of a catalyst material, the hydrocarbon-type
fuel supply unit may be provided, as appropriate, with a
desulfurizing unit or the like. However, as described in Example 2,
the present invention performs reforming at high temperatures in
the co-presence of water and air with the use of a reforming
catalyst comprising platinum having high durability with respect to
sulfur compounds, and this makes it possible to realize a hydrogen
producing apparatus having a desulfurizing unit arranged downstream
of a reforming catalyst. This allows exhaust heat of a reforming
section and heat of a CO shifting section to become available for
desulfurization and therefore enables reduction in the size of the
desulfurization unit in comparison with the desulfurization unit
comprising a zeolite-type adsorbent.
* * * * *